1. Field of the Invention
This invention relates to a multilayer film and more particularly to a multilayer film made of synthetic resin which is used as a plastic film for packing, etc. and which can easily be torn in a certain direction.
2. Brief Description of the Prior Art
As conventional methods for manufacturing a multilayer film of this type, there are described methods for using a resin readily oriented in molecularly in the taking over film when the direction, is being manufactured by extrusion such as a high density polyethylene. This high density polyethylene is conventionally present as a part of the layers forming a multilayer film structure. In order to decrease a so-called blow ratio so that the film can easily be torn in the taking-over direction (vertical direction) and after the film is formed in a membranous form, it is stretched to orient molecules thereof so that the film can easily be torn. Also, a method is already practiced in which a multi-layer film having easy-to-tear characteristics is manufactured by laminating an easy-to-tear film manufactured by the above-mentioned method with other films.
However, even if a film, such as high density polyethylene, which is readily oriented molecularly in the taking-over direction, is used to decrease the blow ratio, the improvement in the easy-to-tear characteristic of a film in the vertical direction is generally insufficient and it is difficult to obtain a film which has an easy-to-tear characteristic sufficient enough to be practically useful. In the case of method wherein a film is stretched after it is formed in a membranous form so that the film can easily be torn in the vertical direction, it is not only required to employ a special apparatus for stretching film, but also the tearing strength in the vertical direction becomes too weak in general to offer an appropriate easiness of vertical tearing.
On the other hand, in the case of a method wherein an easy-to-tear film manufactured beforehand is attached to an other film, unless the tearing strength of the easy-to-tear film used as a raw material is weak enough, it is difficult to facilitate the tearing sufficiently to be practically effective owing to the tear resistance of the aforementioned other film in which it is attached. However, since this type of a weak film is very difficult in respect of attaching treatment, many problems must be overcome before it can be actually put into practice.
The present invention has been accomplished in order to obviate these problems inherent in the prior art.